Distribution and Ultrastructure of Ciliated Sensory Receptors in the Posterior Mantle Epithelium of Dentalium rectius (Mollusca,Scaphopoda)

The scaphopod mantle cavity opens posteriorly via the pavilion, a siphon-like extension of the posterior mantle through which the respiratory currents pass. The pavilion was examined for ciliated sensory cells in Dentalium rectius Carpenter, 1865, using scanning and transmission electron micropscopy. Three types of sensory receptor were distinguished on the basis of number, length and ultrastructure of the associated cilia. Receptors with 2–5 cilia of ? 1.7 μm length lined the pavilion edge. A second type, possessing 1–2 cilia, ? 8.2 μm in length, was found throughout the internal and on part of the external surface of the pavilion. The third receptor type consisted of a rigid bundle of 16–40 cilia with a length of ? 14.4 μm, and was present close to the periphery and at the base of the pavilion near the entrance to the mantle cavity. The structure and distribution of these cells are similar to peripheral chemo- and mechanoreceptors which sample respiratory currents and the surrounding environment in other molluscs, but they may assume a greater functional significance in scaphopods due to the absence of an osphradium in this class.     

Phylogenetic relationships among families of the Scaphopoda (Mollusca)

Phylogenetic relationships among families in the molluscan class Scaphopoda were analysed using morphological characters and cladistic parsimony methods. A maximum parsimony analysis of 34 discrete characters, treated as unordered and equally weighted, from nine ingroup terminal taxa produced a single most parsimonious tree; supplementary analyses of tree length frequency distribution and Bremer support indices indicate a strong phylogenetic signal from the data and moderate to minimally supported clades. The traditional major division of the class, the orders Dentaliida and Gadilida, is supported as both taxa are confirmed as monophyletic clades. Within the Dentaliida, two clades are recognized, the first comprised of the families Dentaliidae and Fustiariidae, the second of the Rhabdidae and Calliodentaliidae; together, these groups comprise a third clade, which has the Gadilinidae as sister. Within the Gadilida, a nested series of relationships is found among [Entalinidae, [Pulsellidae, [Wemersoniellidae, Gadilidae]]]. These results lend cladistic support to earlier hypotheses of shared common ancestry for some families, but are at variance with other previous hypotheses of evolution in the Scaphopoda. Furthermore, analysis of constituent Gadilinidae representatives provide evidence for paraphyly of this family. The relationships supported here provide a working hypothesis that the development of new characters and greater breadth of taxonomic sampling can test, with a suggested primary goal of establishing monophyly at the family level.     

The protonephridial system of the tusk shell, Antalis entalis (Mollusca, Scaphopoda)

The development and microanatomy of the protonephridial system in larvae and postmetamorphic juveniles of Antalis entalis (Dentaliidae) have been examined by means of a semithin serial sectioning and reconstruction technique. One late larval stage has been additionally examined by transmission electron microscopy. The protonephridium appears during larval development and is reduced in the juvenile approximately 13 days after metamorphosis. This is the first unambiguous evidence of a protonephridium in a postlarval mollusc. When fully developed the protonephridium is unique in consisting of two cells only, a terminal cell (=cyrtocyte) and a duct-releasing cell with glandular appearance. The polyciliary terminal cell has several distinct ultrafiltration sites, resembling conditions in bivalve protonephridia. The large duct-releasing cell shows a very large nucleus probably reflecting polyploidy. Its basal infoldings and many mitochondria suggest metabolic activity, the cytoplasm is characterised by many distinct granules. The unique features of the scaphopod protonephridial system are compared with available data on the protonephridia of other molluscan classes. The finding gives additional evidence that protonephridia belong to the ground pattern of the Mollusca. Accepted: 22 January 2001     

Fine structure of the spermatozoa of Crassostrea gigas (Mollusca,Bivalvia)

We describe sperm ultrastructure and acrosome differentiation during spermiogenesis in Crassostrea gigas (Mollusca Bivalvia). The sperm cell is a uniflagellated cell of the primitive type. The head region contains a rounded or conical nucleus surmounted by small acrosome. This organelle consists of a membrane-bound acrosomal granule, the contents of which have a homogeneous density, except in the anterior region, which is positive for PTA. The acrosome also surrounds the perforatorium, which includes oriented fibrillar elements: this is the axial body. The middle piece contains four mitochondria encircling two perpendicular centrioles. The distal centriole is provided with a system of mechanical fixation to the plasma membrane, consisting of nine fibers in radial arrangement. The tail flagellum, about 50 m?m long, contains the usual microtubular axoneme. © 1993 Wiley-Liss, Inc.     

Comparative neuroanatomy of Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda (Mollusca) and its phylogenetic implications

The nervous system of invertebrates is considered to be a very conservative organ system and thus can be helpful to elucidate questions of phylogenetic relationships. Up to now, comparative neuroanatomical studies have been mainly focused on arthropods, where in-depth studies on major brain structures are abundant. In contrast, except for Gastropoda and Cephalopoda, the nervous system of representatives of the second largest phylum of invertebrates, the Mollusca, is as yet hardly investigated. We therefore initiated an immunohistochemical survey to contribute new neuroanatomical data for several molluscan taxa, especially the lesser known Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda, focusing on the cellular architecture and distribution of neurotransmitters in the brain. Antisera against the widespread neuroactive substances FMRFamide and serotonin were used to label subsets of neurons. Both antisera were additionally used in combination with acetylated ??-tubulin and the nuclear marker DAPI. This enables us to describe the morphology of the nervous system at a fine resolution and to compare its cellular architecture between different species of one taxon, as well as between different taxa of mollusks. On the basis of these results, the nervous system of caudofoveates seems to be most highly derived within the so-called basal (non-conchiferan) mollusks, and a monophyly of a clade Aplacophora could not be confirmed. In general, the brain as well as the remaining nervous system of the molluscan taxa investigated shows a great variability, suggesting a deep time origin of the diversification of this prominent protostome clade.     

The Ultrastructure and Functional Morphology of a Captaculum in Graptacme calamus(Mollusca,Scaphopoda)

The structure and function of the captaculum in Graptacme calamuswere studied using light microscopy, videomicrography, and transmission electron microscopy. Graptacme calamusutilizes a longitudinal ciliary band to transport small food particles along the outstretched filament. The head of the captaculum, which bears the alveolar indentation, contains a large ganglion, at least three types of gland cells, circular and longitudinal muscles, and connective tissue. It is likely that gland 1 secretes mucus which is used in the transport of food particles and that gland 2 secretes material which promotes adhesion of the alveolus to the substratum. The function of gland 3 is unknown. Critical observation of the captaculum surface revealed that the cilia of the alveolar region have truncated tips unlike those found on other regions of the captaculum. These alveolar cilia probably aid in the distribution of adhesive substance and in adhesion.     

Muscle development in Antalis entalis (Mollusca,Scaphopoda) and its significance for scaphopod relationships

We applied fluorescence staining of F-actin, confocal laser scanning microscopy, as well as bright-field light microscopy, SEM, and TEM to examine myogenesis in larval and early juvenile stages of the tusk-shell, Antalis entalis. Myogenesis follows a strict bilaterally symmetrical pattern without special larval muscle systems. The paired cephalic and foot retractors appear synchronously in the early trochophore-like larva. In late larvae, both retractors form additional fibers that project into the anterior region, thus enabling retraction of the larval prototroch. These fibers, together with the prototroch, disappear during metamorphosis. The anlagen of the putative foot musculature, mantle retractors, and buccal musculature are formed in late larval stages. The cephalic captacula and their musculature are of postmetamorphic origin. Development of the foot musculature is dramatically pronounced after metamorphosis and results in a dense muscular grid consisting of outer ring, intermediate diagonal, and inner longitudinal fibers. This is in accordance with the proposed function of the foot as a burrowing organ based on muscle-antagonistic activity. The existence of a distinct pair of cephalic retractors, which is also found in basal gastropods and cephalopods, as well as new data on scaphopod shell morphogenesis and recent cladistic analyses, indicate that the Scaphopoda may be more closely related to the Gastropoda and Cephalopoda than to the Bivalvia.     

Fine structure and axonal organization in the buccal ganglia nerves ofAplysia (Mollusca,Gastropoda)

Summary The morphology of theAplysia buccal nerves and connectives has been studied by electron microscopic analysis. In these nerves the fine structure of the elements (connective sheath, glia, axons and their vesicular and cytoplasmatic content) is similar to that of other molluscan nerves. Some features seem to be comparable to other invertebrate groups such as Crustacea and Annelida. The axons have been divided into four classes on the basis of their calibre, and each type has been counted in all the nerves. The number of axons relating to identified buccal neurons is discussed. Finally, some speculations about relationships between buccal ganglia and peripheral regions connected by buccal nerves are proposed.     

Fine structure of the eyes of Onithochiton neglectus (Mollusca: Polyplacophora)

Summary Onithochiton neglectus a common littoral chiton possesses large numbers of small eyes embedded in the outer layer of the shell, the tegmentum. These are arranged in a definite pattern on each shell valve. Each eye lies in a pocket, and is surrounded by pigment laid down in the shell. There is a lens, cup of retina cells and an optic nerve running in an optic canal through the shell. Glial elements are present. The retina cells give rise centrally to a packed array of microvilli, a rhabdom. Cilia are present at the edge of the rhabdom; they have a 9 + 2 arrangement of ciliary filaments and do not appear to be involved in the formation of microvilli. Cells at the periphery of the eye cup give rise to large whorls of membranes, lamellate bodies. These bodies are derived from the membranes of cilia having a 9 + 2 pattern, and form into an extra-cellular space. Nerve processes from the retina cells pass into the optic canal. On the basis of previous work it is thought that the lamellate bodies are also sensory. These structures are discussed in relation to other microvillar and lamellate structures described from photoreceptors.I thank Professor J. E. Morton for his advice in the early stages of this work, and Dr. S. J. Bullivant for the fixation and embedding of material for electron microscopy. To Professor G. A. Horridge I am grateful for advice and the facilities of his laboratory, and to Professor M. S. Laverack, Patricia Holborow and Charles Coleman for much help and encouragement. I am supported by the Science Research Council, and in New Zealand held a Commonwealth Scholarship.     

Fine structure of the subradular Organ of Lepidochitona cinereus (L), (Mollusca,Polyplacophora)

Electron microscopy of the subradular organ of the chiton Lepidochitona cinereus (L) reveals at least three cell types, microvillous, ciliated and mucus-secreting, situated in a single epithelium. The base of the epithelium is abundantly innervated and supplied with muscle cells. The fine structure is consistent with a chemosensory function for the subradular organ.     

Larval and early post-larval shell of three deep-sea Scaphopoda (Mollusca) from the southwest Atlantic

The larval shells of Antalis circumcincta (Watson, 1879) (order Dentaliida), Pertusiconcha callithrix (Dall, 1889) (order Gadilida) and of an undetermined species of uncertain systematic position are described. The material studied comprises mainly samples from deep waters, collected by expeditions along the southeast coast of Brazil. The larval shell of the three taxa matches other types previously described in the literature. Antalis circumcincta and P. callithrix have four regions (protoconch A, protoconch B, teleoconch A, teleoconch B), but differ in dimensions and sculpture from each other, while the undetermined species has three regions (protoconch A, protoconch B, teleoconch B). A morphometric approach combined with a discriminant analysis also indicates that the three taxa are significantly distinct. This study confirms patterns of larval shells at the taxonomic rank of orders but other supraspecific patterns remain uncertain.     

Fine structure of the germinating cells during the spermiogenesis of Arion rufus (Mollusca,Pulmonate Gastropoda)

Changes in spermatozoan ultrastructure have been studied during spermiogenesis of the slug Arion rufus (Gastropoda, Pulmonata, Stylommatophora). The ovotestis was investigated during the male stage, definite by the presence of spermatozoa. Some peculiar characteristics are shown by early spermatids: Around the nucleus, the nuclear envelope presents two thick layers located on opposite sides, the apical and basal plates, that will determine the antero-posterior axis of the spermatid. The chromatin, first dispersed throughout the nucleoplasm gives later on thick filaments which become attached over the inner surface of these plates. The chromatin filaments are then arranged parallel to the antero-posterior axis as the nucleus elongates. The position of the plates determines the antero-posterior axis of the spermatid. In the mature spermatozoa, the chromatin is more condensed and the nucleus presents an helical organization. The acrosome and flagellum are respectively attached externally to the center of the apical and basal plates. The acrosome consists of a membrane-bound vesicle and forms a column of homogeneous material. In the middle piece, the mitochondria have been transformed into a mitochondrial derivate by the way of a complicated metamorphosis. The axoneme is surrounded by three mitochondrial helices but only one of them contains glycogene granules. © 1996 Wiley-Liss, Inc.     

Fine structure and function of the digestive cell of Sepia officinalis (Mollusca: Cephalopoda)

The post–embryonic development of the digestive gland has been studied by light and electron microscopy on animals reared in the laboratory. The gland reaches its "adult" structure around the end of the first month after hatching.
The infrastructural changes of the digestive gland have been followed during digestion at various ages (5,10,20,30 days and "adult"); ferritin was used for a tracer to study absorption.
These experiments have produced evidence of endocytosis intake of large proteins and further intracellular digestion inside the digestive cell. The residues of digestion are excreted in a "brown body" while the metabolites are used in new syntheses for the cell itself. These results demonstrate that "ancestral" digestive processes have been kept in at least one cephalopod.     

The phylogeny and classification of Scaphopoda (Mollusca): an assessment of current resolution and cladistic reanalysis

The first cladistic analysis of phylogeny in the class Scaphopoda (Steiner 1992a,1996) examined relationships among family and selected sub-family taxa using morphological data. A preferred/ consensus tree of relationships illustrated monophyly of the orders Dentaliida and Gadilida, partial resolution among dentaliid families, and complete resolution among gadilid taxa. However, several alternative replications of the analysis, including use of a revised data matrix, did not produce the reported tree number or level of resolution; in all cases, monophyly of the Dentaliida was not supported by strict consensus of resultant parsimonious trees. Reanalysis, using unordered characters and outgroup rooting, only clearly resolves monophyly of the Gadilida and the sister relationship of the Entalinidae with the remaining gadilid families. These analyses emphasize the need for more comparative data and thorough parsimony analysis in scaphopod cladistic phylogenetics, as relationships in this class are still some way from resolution.     

Fine structure of the early stages of spermatogenesis in the Pacific oyster, Crassostrea gigas (Mollusca, Bivalvia)

The aim of this study is to describe the early stages of spermatogenesis of the Pacific oyster Crassostrea gigas using both light and electron microscopy. The gonad is formed by gonadal tubules invaginated in a connective tissue constituting a storage tissue. Myoepithelial cells surround each gonadal tubule and are associated with an acellular matrix delimiting the outer part of the tubule, the inner part is composed by intragonadal somatic cells associated with germinal lineage. Two types of spermatogonia are identified, where type I spermatogonia (Spg I) are large, scarce and pale cells leaned against the base of the tubule (nuclear diameter: 5.5+/-0.5 microm). Type II spermatogonia (Spg II) are clustered and dark cells which appear smaller than type I (nuclear diameter: 4.3+/-0.3 microm). The aspect of nuage-like material in cytoplasm is described from pale spermatogonia to primary spermatocytes (nuclear diameter: pachytene 3.6+/-0.3 microm, diplotene 3.4+/-0.3 microm), while no structure related to a chromatoid body was observed in oyster spermatocytes and spermatids.     

The expression of an engrailed protein during embryonic shell formation of the tusk-shell, Antalis entalis (Mollusca, Scaphopoda)

SUMMARY This study presents the first detailed account of the larval and early post-metamorphic development of a scaphopod species, Antalis entalis , since 1883. Special reference is given to the expression pattern of an engrailed protein during the formation of the embryonic (protoconch) and adult shell (teleoconch). We found that in the trochophore-like larva the engrailed protein is expressed in shell-secreting cells at the margin of the protoconch close to the mantle edge. During metamorphosis the growth of the protoconch and expression of the engrailed protein along its margin stop and the teleoconch starts to form. These data suggest a different genetic background regarding protoconch and teleoconch formation in the Scaphopoda and possibly all Conchifera, thus inferring a different evolutionary origin of both organs. The single anlage of the scaphopod protoconch contradicts earlier hypotheses of a monophyletic taxon Diasoma (Scaphopoda + Bivalvia), which has been mainly based on the assumption of a primarily bilobed shell in both taxa. Comparative data on engrailed expression patterns suggest nervous system patterning as the basic function of engrailed in the Bilateria. However, there are several independent gain-of-function events, namely segment compartmentation in the Annelida and Arthropoda, protoconch formation in the Mollusca, skeletogenesis in the Echinodermata, and limb formation in vertebrates. These findings provide further evidence that homologous genes may act in very different pathways of bilaterian body plan formation in various animal phyla.     

Fine structure of the mineralized teeth of the chiton Acanthopleura echinata (Mollusca: Polyplacophora)

The major lateral teeth of the chiton Acanthopleura echinata are composite structures composed of three distinct mineral zones: a posterior layer of magnetite; a thin band of lepidocrocite just anterior to this; and apatite throughout the core and anterior regions of the cusp. Biomineralization in these teeth is a matrix-mediated process, in which the minerals are deposited around fibers, with the different biominerals described as occupying architecturally discrete compartments. In this study, a range of scanning electron microscopes was utilized to undertake a detailed in situ investigation of the fine structure of the major lateral teeth. The arrangement of the organic and biomineral components of the tooth is similar throughout the three zones, having no discrete borders between them, and with crystallites of each mineral phase extending into the adjacent mineral zone. Along the posterior surface of the tooth, the organic fibers are arranged in a series of fine parallel lines, but just within the periphery their appearance takes on a "fish scale"-like pattern, reflective of the cross section of a series of units that are overlaid, and offset from each other, in adjacent rows. The units are approximately 2 microm wide and 0.6 microm thick and comprise biomineral plates separated by organic fibers. Two types of subunits make up each "fish scale": one is elongate and curved and forms a trough, in which the other, rod-like unit, is nestled. Adjacent rod and trough units are aligned into large sheets that define the fracture plane of the tooth. The alignment of the plates of rod-trough units is complex and exhibits extreme spatial variation within the tooth cusp. Close to the posterior surface the plates are essentially horizontal and lie in a lateromedial plane, while anteriorly they are almost vertical and lie in the posteroanterior plane. An understanding of the fine structure of the mineralized teeth of chitons, and of the relationship between the organic and mineral components, provides a new insight into biomineralization mechanisms and controls.